1. Field of the Invention
The present invention relates to a telecommunications apparatus, and more particularly, to a telecommunications apparatus of a type typically used in telecommunications stations to carry out high-speed telecommunications, in which a plurality of shell-type plug-in units are aligned inside a subrack which is mounted in a rack.
2. Description of Related Art
Together with the growing importance of telecommunications in recent years there is a growing need for improvements in the quality of the telecommunications equipment used in station buildings and the like. One such improvement in quality has to do with lowering the amount of electromagnetic interference (EMI) radiated from such equipment, as well as reducing the electromagnetic susceptibility (EMS) of such equipment to external EMI. Taken together, these measures are an index of the electromagnetic compatibility, or EMC, of the equipment.
Additionally, the transmission capacity of telecommunications equipment continues to increase. Such increases in transmission capacity require increasing the transmission frequencies of the oscillators mounted in the plug-in units. At present the frequencies are on the order of 10 GHz, though the next generation of telecommunications equipment should see frequencies on the order of 40 GHz.
Ordinarily, conventional telecommunications equipment is designed so that a connector on an end of the plug-in unit that is inserted into the subrack is connected to a connector on a back wiring board of the subrack and mounted in the subrack thereby.
Conventionally, a cover or shield is provided on the front of the subrack into which the plug-in units are so closely packed and inserted in order to reduce EMI emissions. Thus measures to improve EMC are carried out on a per-subrack-unit basis, one subrack at a time.
Indeed, a variety of telecommunications equipment has been developed in which measures to improve EMC have been conducted on the subrack level. These types of telecommunications equipment use a so-called shell construction for the plug-in unit, in which the printed board on which the circuits are mounted is enclosed within a metal casing.
The shell-type plug-in unit is inserted into the subrack, so an exposed connector is provided at an end of the plug-in unit that is inserted into the subrack. This exposed connector thus lacks adequate EMC, and thus it is necessary to shield the insertion end of the plug-in unit when the plug-in unit is inserted in the subrack, that is, when the plug-in unit connector is connected to the back wiring board connector.
FIGS. 1A and 1B are diagrams in which a plug-in unit for a telecommunications apparatus previously suggested by the applicant is shown in a state just prior to mounting in a subrack and in a state in which the plug-in unit is mounted in the subrack, respectively.
As shown in the diagrams, a plug-in unit 1 is constructed such that a leaf spring member 3 surrounds a connector 2. A subrack 10 is constructed so that a metal frame member 12 is fixedly mounted on a front surface of a back wiring board 11, with ribs 13 positioned along both sides of a connector 14. In a state in which the plug-in unit 1 is mounted in the subrack 10 as shown in FIG. 1B, the leaf spring members 3 contact the ribs 13, thus fully shielding the inserted end of the plug-in unit 1 over its entire periphery.
Here, the permissible slot length allowed to that portion shielded from electromagnetic radiation greatly effects the effectiveness of that shield. Shield effectiveness in the case of a slot is expressed as S=20 log (wavelength÷(2×slot length)) dB. For example, in the case of a telecommunications apparatus having a 40 GHz oscillator, the wavelength of the signal generated by the oscillator is 7.5 mm. Accordingly, a slot length equal to 1/20 of that wavelength (or 0.37 mm) would have a shielding effect of 20 dB, a slot length equal to 1/30 of that wavelength (0.25 mm) would have a shielding effect of 23 dB, and a slot length equal to 1/40 of that wavelength (0.1875 mm) would have a shielding effect of 26 dB. Thus, as can be appreciated, the smaller the length of the slot the greater the shielding effect provided. Additionally, because the shielding effect is a function of the wavelength, for a given slot length the shielding effect will vary with the wavelength of the generated signal, so for example a 1.5 mm slot is permissible in order to attain a shielding effect of 20 dB with a wavelength of 10 GHz but the same slot length of 1.5 mm produces a shielding effect of only 8 dB when the wavelength is 40 GHz.
Additionally, the shield effect is also related to the number of slots, and is expressed asS=−20 log√{square root over (n)},
where n is the number of slots, so that, for example, when there are two slots the shielding effect declines by −3 dB. Thus, where the shielding effect is 20 dB with one slot, the additional slot reduces the shielding effect to 17 dB.
As described above, in order to increase the shield effect it is necessary to make the length of the slot 1/30 to 1/40 of the wavelength of the generated signals, which indicates that, in the case of ultra-high signals on the order of 40 GHz, essentially no gap at all is permissible.
However, even the best machining of the surfaces of the ribs 13 leaves an undulation thereon. Such undulations weaken the contact between the leaf spring members 3 and the ribs 13, that is, create a gap between the leaf spring members 3 and the surfaces of the ribs 13.
As a result, telecommunications equipment having the structure shown in FIGS. 1A and 1B is not capable of providing adequate EMC where the oscillators generate signals on the order of 40 GHz, and hence are not suitable for the next generation of telecommunications equipment.